1. Photosynthesis 8th Grade Science
Standard 2.4. Photosynthesis and cellular respiration are important processes by which energy is acquired and utilized by organisms

2. Video content
1. Explain the process of photosynthesis and where it occurs in a cell
2. Discuss why photosynthesis is an important process on Earth
3. Introduction to the Photosynthesis Lab and discussion of the procedure
4. Assignment to complete after watching the video: Predict the results that you will see in the experiment.

3. Formula for photosynthesis
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

4. Why is photosynthesis important?
Photosynthesis is the most important process occurring on Earth. It fuels ecosystems and replenishes our atmosphere with free oxygen.
Photosynthesis converts light energy into the chemical energy of sugars and other organic compounds. Oxygen (O2) is a byproduct of photosynthesis and is released into the atmosphere. Oxygen is necessary for respiring organisms.

5. Where does photosynthesis occur
Photosynthesis happens in the chloroplast. Within this cell organelle is the chlorophyll that captures the light from the Sun. Mesophyll cells are specialized for photosynthesis. These cells in the middle of the leaf contain many chloroplasts. Chlorophyll molecules embedded in the thylakoid membrane absorb light energy.

6. Measuring the rate of photosynthesis
The rate of photosynthesis can be determined by measuring either:
The production of O2
The consumption of CO2
In the case of photosynthesis, we can either measure the rate of reaction by the loss of carbon dioxide or the accumulation of sugars or oxygen. In this lab we will use a system that measures the accumulation of oxygen.

7. Lab introduction
Oxygen bubbles are usually trapped in the air spaces of the spongy mesophyll in the plant leaf. By creating a vacuum in this experimental procedure, the air bubbles can be drawn out of the spongy mesophyll, which is then infiltrated by the surrounding solution. This allows the leaf disks to sink in the experimental solution. If the solution has bicarbonate ions and enough light, the leaf disk will begin to produce sugars and oxygen through the process of photosynthesis. Oxygen collects in the leaf as photosynthesis progresses, causing the leaf disks to again float. This procedure indirectly measures the net rate of photosynthesis.

8. procedure
1. Prepare 300 mL of 0.2% bicarbonate solution for each trial. This can be done by dissolving 1/8 teaspoon of baking soda in 300 mL of water. The bicarbonate will serve as a source of carbon dioxide for the leaf disks while they are in the solution.
2. Pour the bicarbonate solution into a clear plastic cup to a depth of about 3 cm. Labe this cup “with CO2”. Fill a second cup with only water to be used as a control group. Label this cup “without CO2”.

9. Procedure continued
3. Using a pipette, add 1 drop of dilute liquid soap solution to the solution in each cup. The soap acts as a surfactant or “wetting agent” — it wets the hydrophobic surface of the leaf allowing the solution to be drawn into the leaf and enabling the leaf disks to sink in the fluid.
4. Using a hole punch, cut out 10 leaf disks for each cup from the provided spinach leaves.

10. Procedure continued
5. Draw the gases out of the spongy mesophyll tissue and infiltrate the leaves with the sodium bicarbonate solution. To do this:
Remove the plunger from both syringes and place 10 leaf disks into each syringe barrel.
Replace the plunger and push it in carefully until only a small volume of air and leaf disks remain in the barrel.
Put a small volume of sodium bicarbonate solution into one syringe and a small volume of water into the other syringe. Tap the syringe to suspend the leaf disks in the solution.
Create the vacuum to draw the air out of the leaf by holding a finger over the narrow syringe opening while drawing back the plunger. Hold this vacuum for about 10 seconds. The solution will infiltrate the air spaces in the leaf disk, causing the leaf disks to sink in the syringe. Repeat this procedure 2-3 times in order to get the disks to sink.

11. Procedure continued
6. Pour the disks and the solution from the syringe into the appropriate clear plastic cup. Disks infiltrated with the bicarbonate solution go in the “with CO2” cup and the disks infiltrated with just water go in the “without CO2” cup.
7. Place both cups under the light source and start the timer. At the end of each minute, record the number of floating disks. Continue until all of the disks are floating in the cup with the bicarbonate solution.

12. prediction
What do you think will happen to the spinach leaves in the water without carbon dioxide?
What do you think will happen to the spinach leaves in the water with carbon dioxide?
Explain why you believe this.
Bring your written prediction to the next class period.